This invention relates to foams in general, and more particularly, to foams prepared from a blend of syndiotactic polypropylenes and foamable thermoplastic polymer resins.
Foams are used in a number of applications, including thermal insulation, packaging, and the formation of molded articles such as cups, and trays. Depending upon the end-use of the foam, it is desirable that the foam exhibits particular properties or combinations of properties. For example, when utilized as an insulating material or cushion packaging material, expansion of the foam to low densities is quite desirable.
In addition, foams having a high temperature of distortion are desirable for a number of applications, including insulation in high temperature environments. Depending upon the particular use of such insulating foams, it is also desirable that such foams be flexible in addition to having a high temperature of distortion. The degree of flexibility and the heat distortion requirements vary depending on the end-use of the insulating foam. For example, in some automotive uses or for insulation of hot-water pipes, a flexible foam having a heat distortion temperature greater than 90xc2x0 C., or greater than 110xc2x0 C. to 120xc2x0 C. in some instances, is required. In addition, in order to be in compliance with Underwriters Laboratory Test UL 1191 Appendix A, foams used as fillers in personal flotation devices need to withstand temperatures of 60xc2x0 C. for a prolonged period of time in addition to being very soft and flexible.
However, it is difficult to achieve the properties of flexibility (that is, low modulus of elasticity) and high heat distortion temperature in the same foam. Typically, the flexibility of a given resin (that is, modulus of elasticity) and the heat distortion temperature of that resin are both directly related to the melting point of the resin. In other words, a resin having a low modulus of elasticity (that is, high flexibility) typically requires that the resin have a lower melting point whereas a resin having a high temperature of distortion typically requires that the resin have a higher melting point. In addition, even if properties of flexibility and high heat distortion temperature are found in the same resin or resin blend, the resin or resin blend may not be amenable to foaming, or to foaming by a convenient process such as the extrusion process. Branched-polyolefin resins prepared by the high-pressure-free-radical process are foamable to a flexible foam by extrusion, but the foam lacks the temperature resistance for certain applications. Examples of branched-polyolefin resins include low-density polyethylene homopolymer resins having densities in the range of from 0.915 g/cm3 to 0.932 g/cm3 and copolymers of ethylene with a vinyl ester such as vinyl acetate and methyl acrylate. Linear polyolefin homopolymer resins prepared by a catalytic process (using for example, Ziegler Natta or metallocene catalysts), such as high-density polyethylene and isotactic polypropylene (iPP) resins, have a relatively high heat distortion temperature but are difficult to foam by the extrusion process. In addition, the foams made from such stiff homopolymer resins lack flexibility. A less stiff linear copolymer resin can be made by the catalytic process but the resin suffers from the same lack of foamability as do the homopolymers. The use temperature of a polyolefin resin foam can be increased by cross-linking. For example, a cross-linked foam prepared from a low-density polyethylene resin may be used at a temperature higher than for an uncross-linked foam but the use temperature, which is less than 100xc2x0 C., is not sufficiently high for some applications. In addition, a cross-linked foam is costly to manufacture and is not recyclable.
Foams prepared from a blend of high melting polyolefin resins (for example, high density polyethylene and iPP) and low density polyethylene (LDPE) resin are difficult to expand to a low density foam by extrusion processes, since the foam expansion relies on the freezing transition of the high melting resin which has a poor foamability. When applied to the cross-linking approach, such a blend causes another type of difficulty. In that process, a foamable composition is extruded into a sheet at a low temperature where the blowing agent and the cross-linking agent remain substantially unactivated. Often, the required processing temperature for a high melting linear polyolefin exceeds the tolerable temperature for the blowing agent and the cross-linking agent and may prematurely activate them.
However, depending on the particular end-use, it is not always desirable that an insulation foam be flexible. Rigid foams having high temperatures of distortion are also desirable. Rigid insulation foams are often prepared from alkyl aromatic polymers, such as polystyrene, which due to environmental concerns, are increasingly expanded with carbon dioxide. However, low density foams, such as polystyrene, which are expanded with carbon dioxide exhibit a small cell size.
However, in order that a rigid foam be readily formed from the convenient extrusion process and be easily fabricated, it is necessary that the foam has an enlarged cell-size. A foam having a small cell size is not only difficult to extrude to a large cross-section but is also difficult to fabricate (for example, slice and cut to final shapes). In order for ready fabrication, it is desirable that the foam have a cell size greater than 0.4 mm.
There have been various attempts to prepare alkyl aromatic foams having an enlarged cell size by incorporating various additives which enlarge the cell size (see, for example, U.S. Pat. Nos. 4,229,396 and 5,489,407). However, typical cell-enlarging additives are difficult to feed into the extruder and tend to affect the heat distortion temperature of the foam product.
In addition to insulation foams having high temperatures of distortion, which are either 1) flexible or 2) rigid with enlarged cell size, flexible foams which are made from thermoplastic polymers having a Tg above 0xc2x0 C. are desirable for end-uses which require noise and vibration damping as well as for cushion packaging. When used for cushion packaging or vibration damping, a flexible foam protects the article by absorbing the impact and vibration energy in its cell structure. The energy is absorbed both in the gas and polymer phase. A foam having cell walls that irreversibly dissipate the mechanical energy into heat is desirable. A polymer resin dissipates mechanical energy most effectively at the glass transition temperature (Tg) of the resin (see, for example, Properties of Polymers, third edition, Chapter 14, xe2x80x9cAcoustic Properties,xe2x80x9d ed. By D. W. Van Krevelen, Elsevier, Amserdam-London, New York-Tokyo, 1990). Most conventional polyolefin resins such as polyethylene and polypropylene are flexible, but have a relatively low Tg, that is, below 0xc2x0 C. and are, therefore, not useful for cushion packaging or vibration damping end-uses.
Therefore, there remains a need in the art for 1) flexible foams having a high temperature of distortion; 2) rigid alkyl aromatic foams having a high heat of distortion and enlarged cell size which are conveniently made and wherein the heat of distortion is stable; and 3) flexible thermoplastic foams made from a thermoplastic polymer having a Tg above 0xc2x0 C.
Those needs are met by the present invention. Thus, the present invention provides polymer foams prepared from a blend of a syndiotactic polypropylene (sPP) resin and a foamable thermoplastic polymer resin.
Thus, in a first embodiment of the present invention, there are provided polymer foams prepared from a blend of a sPP resin and a flexible thermoplastic polymer resin which are flexible and have a high temperature of distortion. The polymer foams according to the first embodiment of the present invention are useful as insulating foams in high temperature environments wherein a flexible foam is desired, such as some automotive uses and insulation of hot-water pipes. In addition, since sPP resin has a Tg of 4xc2x0 C., the polymer foams according to the first embodiment of the present invention are also suitable as cushion packaging or in noise or vibration damping products.
Typical blended polymer foams according to the first embodiment are as follows:
a blended polymer foam, comprising: a) from 0.1 percent to 60 percent by weight of a sPP resin; and b) from 40 percent to 99.9 percent by weight of a flexible thermoplastic polymer resin;
a blended polymer foam, comprising: a) from 10 percent to 50 percent by weight of a sPP resin; and b) from 50 percent to 90 percent by weight of a flexible thermoplastic polymer resin; and
a blended polymer foam, comprising: a) from 30 percent to 50 percent by weight of a sPP resin; and b) from 50 percent to 70 percent by weight of a flexible thermoplastic polymer resin.
In a second embodiment of the present invention, there are provided polymer foams prepared from a blend of a sPP resin and a rigid thermoplastic polymer resin which are rigid, have high temperatures of distortion, and have enlarged cell size. The sPP resin additive, which enlarges the cell size of the rigid thermoplastic polymer foams, is easily fed into the extruder, and does not affect the heat distortion temperature of the foam.